Eduardo J. H. Lee

Contact and edge effects in graphene devices

Electrical transport studies on graphene have been focused mainly on the linear dispersion region around the Fermi level and, in particular, on the effects associated with the quasiparticles in graphene behaving as relativistic particles known as Dirac fermions. However, some theoretical work has suggested that several features of electron transport in graphene are better described by conventional semiconductor physics. Here we use scanning photocurrent microscopy to explore the impact of electrical contacts and sheet edges on charge transport through graphene devices. The photocurrent distribution reveals the presence of potential steps that act as transport barriers at the metal contacts. Modulations in the electrical potential within the graphene sheets are also observed. Moreover, we find that the transition from the p- to n-type regime induced by electrostatic gating does not occur homogeneously within the sheets. Instead, at low carrier densities we observe the formation of p-type conducting edges surrounding a central n-type channel.

Spin-resolved Andreev levels and parity crossings in hybrid superconductor-semiconductor nanostructures

The physics and operating principles of hybrid superconductor-semiconductor devices rest ultimately on the magnetic properties of their elementary subgap excitations, usually called Andreev levels. Here we report a direct measurement of the Zeeman effect on the Andreev levels of a semiconductor quantum dot with large electron g-factor, strongly coupled to a conventional superconductor with a large critical magnetic field. This material combination allows spin degeneracy to be lifted without destroying superconductivity. We show that a spin-split Andreev level crossing the Fermi energy results in a quantum phase transition to a spin-polarized state, which implies a change in the fermionic parity of the system. This crossing manifests itself as a zero-bias conductance anomaly at finite magnetic field with properties that resemble those expected for Majorana modes in a topological superconductor. Although this resemblance is understood without evoking topological superconductivity, the observed parity transitions could be regarded as precursors of Majorana modes in the long-wire limit.

Zero-bias anomaly in a nanowire quantum dot coupled to superconductors.

We studied the low-energy states of spin-1/2 quantum dots defined in InAs/InP nanowires and coupled to aluminum superconducting leads. By varying the superconducting gap Δ with a magnetic field B we investigated the transition from strong coupling Δ << T(K) to weak-coupling Δ >> T(K), where T(K) is the Kondo temperature. Below the critical field, we observe a persisting zero-bias Kondo resonance that vanishes only for low B or higher temperatures, leaving the room to more robust subgap structures at bias voltages between Δ and 2Δ. For strong and approximately symmetric tunnel couplings, a Josephson supercurrent is observed in addition to the Kondo peak. We ascribe the coexistence of a Kondo resonance and a superconducting gap to a significant density of intragap quasiparticle states, and the finite-bias subgap structures to tunneling through Shiba states. Our results, supported by numerical calculations, own relevance also in relation to tunnel-spectroscopy experiments aiming at the observation of Majorana fermions in hybrid nanostructures.

Gate dependent photocurrents at a graphene p-n junction

We have used scanning photocurrent microscopy to explore the electronic characteristics of a graphene p-n junction fabricated by local chemical doping of a graphene sheet. The photocurrent signal at the junction was found to be most prominent for gate voltages between the two Dirac points of the oppositely doped graphene regions. The gate dependence of this signal agrees well with simulations based upon the Fermi level difference between the two differently doped sections. It is concluded that the photocurrent maps are dominated by the built-in electric field, with only a minor photothermoelectric contribution.

Electrical Properties and Photoconductivity of Stacked‐Graphene Carbon Nanotubes

Stacked-graphene carbon nanotubes (SG-CNTs) obtained via pyrolysis of columnar superstructures of polycyclic aromatic hydrocarbons are studied by electrical transport measurements and scanning photocurrent microscopy. The charge transport in individual SG-CNTs (see figure) is found to be dominated by variable range hopping. Their photoconductive properties render the SG-CNTs of interest for photodetector applications.

Noninvasive metal contacts in chemically derived graphene devices

We study the properties of gold contacts on chemically derived graphene devices by scanning photocurrent microscopy and gate-dependent electrical transport measurements. In the as-fabricated devices, negligible potential barriers are found at the gold/graphene interface, reflecting the noninvasive character of the contacts. Device annealing above 300 °C leads to the formation of potential barriers at the contacts concomitant with metal-induced p-type doping of the sheet as a consequence of the diffusion of gold from the electrodes. The transfer characteristics of the chemically derived graphene devices point toward the suppression of Klein tunneling in this material.

Strong p‐Type Doping of Individual Carbon Nanotubes by Prussian Blue Functionalization

Keywords: carbon nanotubes ; doping ; electrodeposition ; Prussian blue ; Field-Effect Transistors ; Electrochemical Preparation Method ; Modified Electrodes ; Charge-Transfer ; Single ; Oxidation ; Devices ; Bundles ; Composites ; Deposition Reference EPFL-ARTICLE-160520doi:10.1002/smll.200800803View record in Web of Science Record created on 2010-11-30, modified on 2017-05-12

Effect of Stacking Order on the Electric-Field Induced Carrier Modulation in Graphene Bilayers

When planar graphene sheets are stacked on top of each other, the electronic structure of the system varies with the position of the subsequent sublattice atoms. Here, we employ scanning photocurrent microscopy to study the disparity in the behavior of charge carriers for two different stacking configurations. It has been found that deviation from the regular Bernal stacking decouples the sheets from each other, which imparts effective electrostatic screening of the farther layer from the underlying backgate. Electrochemical top-gating is demonstrated as a means to selectively tune the charge carrier density in the decoupled upper layer.

Scaling of subgap excitations in a superconductor-semiconductor nanowire quantum dot

We acknowledge financial support from the European Research Council under the ERC Agreement No. 280043, and from the Spanish Ministry of Economy and Competitiveness through Grants No. FIS2012-33521 and No. FIS2015-64654-P (MINECO/FEDER). R.Ž. acknowledges the support of the Slovenian Research Agency (ARRS) through Grants No. P1-0044 and No. J1-7259. MINECO/ICTI2013-2016/FIS2015-64654-P

Split-Channel Ballistic Transport in an InSb Nanowire

We report an experimental study of one-dimensional (1D) electronic transport in an InSb semiconducting nanowire. A total of three bottom gates are used to locally deplete the nanowire, creating a ballistic quantum point contact with only a few conducting channels. In a magnetic field, the Zeeman splitting of the corresponding 1D sub-bands is revealed by the emergence of conductance plateaus at multiples of e2/h, yet we find a quantized conductance pattern largely dependent on the configuration of voltages applied to the bottom gates. In particular, we can make the first plateau disappear, leaving a first conductance step of 2 e2/ h, which is indicative of a remarkable 2-fold sub-band degeneracy that can persist up to several tesla. For certain gate voltage settings, we also observe the presence of discrete resonant states producing conductance features that can resemble those expected from the opening of a helical gap in the sub-band structure. We explain our experimental findings through the formation of two spatially separated 1D conduction channels.